For example, the feature jar "org.apache.directory.studio.ldapbrowser.feature_2.0.0.v20120224.jar" contains a feature.xml file referencing the org.apache.commons.codec plugin 1.6 :
...
<plugin
id="org.apache.commons.codec"
download-size="0"
install-size="0"
version="1.6"
unpack="false"/>
...
But such version cannot be found in the site.xml file, as it describes this :